9.8.1 Linked Genes

Question 1

linked genes

Answer 1

• Review: Genes on separate chromosomes assort independently of one another during gamete formation. For two genes that code for two different traits, the offspring that result from this independent assortment will display a probable 9:3:3:1 phenotypic ratio.
• Two linked genes can produce only two kinds of gametes. Two unlinked genes can produce four kinds of gametes.
• Linked genes cannot independently assort from one another, and the resulting offspring do not display a 9:3:3:1 phenotypic ratio.
- Our 23 pairs of chromosomes contain thousands of genes, and many genes must share the same chromosome.
- Linked genes are located on the same chromosome.
- Review: The alleles of genes that are on separate
chromosomes (unlinked genes) assort independently of one another during meiosis.
- Linked genes do not assort independently. For example, the alleles for eye color and wing shape in a fly are located on the same chromosome. A fly with a ppBB genotype can only form pB gametes, and a PPbb fly can only form Pb gametes. When the two mate, the resulting generation can make only pB or Pb gametes because the unlinked genes cannot assort independently.
- Review: If the two genes were located on different
chromosomes, a cross between two heterozygous individuals (PpBb) in the F1 would yield an expected phenotypic ratio of 9:3:3:1 in the F2 generation. Each F1 heterozygote will produce four different gametes—PB, Pb, pB, and pb. The gametes would randomly combine, producing four phenotypes in the 9:3:3:1 ratio.
- For linked genes, the heterozygotes only produce two kinds of gametes. The composition of the gametes depends on the allelic combinations in the parent (P) generation. The p and B alleles occurred together in one of the parents. The P and b alleles occurred together in the other parent. The F1 individuals produce gametes with the same combinations—pB and Pb.
- A Punnett square of the F1 gametes is shown on the left. The expected outcome in the F2 generation is a 1:2:1 ratio of phenotypes. One-fourth are expected to be pink and normal, half will be red normal, and one-fourth will be red and blistery

Question 2

Which one of the following represents the gametes that a blistery-winged individual can produce?

Answer 2

• Pb only

Question 3

How do we know that there must be more than one gene on a chromosome?

Answer 3

• There are more genes than there are chromosomes.

Question 4

Suppose a pink-eyed, blister-winged fruit fly is crossed with a wild type (normal) fly. These two traits are very closely linked on the same chromosome. Match the list of genotypes with the expected ratio 1:2:1 in the F2 offspring?

Answer 4

• ppbb:PpBb:PPBB

Question 5

A pink-eyed individual fruit fly is crossed with a blistery-wing individual (parental cross). These two traits are very closely linked on the same chromosome. Which one of the following offspring is a possible result in the F2?

Answer 5

• PPbb

Question 6

Linkage

Answer 6

• decreases genetic variation

Question 7

Trace the inheritance pattern of two linked genes in fruit flies. Red eye color (P) is dominant to pink (p). Normal wings (B) are dominant to blistery (b). If two heterozygous flies are crossed, what phenotypic ratio represents the most reasonable one expected in the offspring?

Answer 7

1/2 red normal
1/4 pink normal
1/4 red blistery

Question 8

In the process of meiosis, ________________ sort independently.

Answer 8

• homologous chromosome pairs

Question 9

An individual of the genotype AABB is crossed with an aabb genotype. The F1 is test crossed. What percent of the test cross progeny will be AaBb if the two genes are unlinked and completely linked, respectively?

Answer 9

• 25%, 50%

Question 10

Which of the following is true regarding Mendel’s experiments with pea plants?

Answer 10

• The inheritance factors he chose to examine were located on separate chromosomes.

Question 11

All of the alleles that occur on a given chromosome are said to form a(n) __________ group.

Answer 11

• linkage

Question 12

If blonde hair and blue eyes are consistently inherited together, the best explanation would be that

Answer 12

• The genes are linked on the same chromosome.

Question 13

What are linked genes?

Answer 13

• Genes that are located on the same chromosome.

Question 14

Suppose a pink-eyed, blister-winged fruit fly is crossed with a wild type (normal) fly. These two traits are very closely linked on the same chromosome. Which of the following genotypes is unexpected in the F2 offspring?

Answer 14

• ppBB

Question 15

Which one of the following provides evidence that some genes are linked?

Answer 15

• In fruit flies, two parent flies that are heterozygous for two different characteristics produce offspring with a 1:2:1 phenotypic ratio.

Question 16

If a recessive gene for one trait is linked to a dominant gene for a second trait, which one of the following is true?

Answer 16

• It is impossible for the recessive phenotype for both traits to occur simultaneously